Such a weird design. I have no other explanation.

This fidget femur uses a print-in-place ovoid joint to create and unique tactile movement. This joint type is stronger than the interlocking double ring joints used in many articulated designs, so this design is more suitable for use as a fidget or children’s toy.

See here for video: https://youtu.be/br1-jSqgU18

Like a “ball and socket” joint, the oval-shaped ovoid from one segment is held in place by a similarly shaped socket in the neighbouring segment. Varying the relative dimensions of the ovoid and socket allows for precise control of the movement between each segment. In this design, lateral side-to-side movement is maximized, but rotation and up/down movement is slightly restricted. You may get some small adhesions between segments. Simply apply a bit of force to move the joint the first time, and it will be fine after that.